Hi Mark,
Ben gave a good answer about the limited risk to the batteries themselves. But if you do combine two banks at significantly different states of charge, you'll lose a significant amount of energy in the process. Since the receiving (low) bank needs a noticably higher voltage to accept anything more than a surface charge, most of the energy flowing out of the sending (high) bank gets lost along the way.
I learned this years ago with an old truck and a loose battery from the garage. I couldn't get another car close enough to jump the truck's dead (reading about 10.5V) battery, so I figured that I would just hook up a fully charged loose battery to get the truck started. I figured that if I waited a few minutes after connecting them, the dead battery would get a little bit of a base charge to start the process. About 30 minutes later, both batteries were too low (both batteries below 11V) to crank the engine.
I carried the loose battery back to the garage, put it on the charger and tried again the next day. This time I cranked the engine as soon as I made the connection and the truck fired right up. The "dead" battery charged up with the alternator and lasted a couple more years of periodic starting/charging before it died completely from sitting a few months at less than 10V.
I had mistakenly figured that the batteries would average out closer to 12V (10.5 + 12.8/2 = 11.6V) but the result was nowhere near that good. The charged battery was actually bigger than the one in the truck so I was surprised that the receiving battery appeared to "catch" less than 1/2 the energy that came out of the charged one.
We see the same thing start to happen when one battery goes bad in a larger parallel bank. The bad battery draws down the other batteries much more than the others can bring the bad one up.
I know that you know that I think that a single bank is better than 2 similar banks. The larger bank is less affected by Peukert's effect and by keeping them combined, you get a small amount of extra range compared to the range of each bank used sequentially. But the difference is small, so there is no real harm in operating them independantly. As usual, it is a matter of personal preference.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "acsarfkram" <acsarfkram@...> wrote:
>
> Hi Pat,
>
> Thanks for the response. I have no intention of combining them, I am a proponent of two separate banks as a safety/redundancy matter. The switching issue has been something that I have been thinking about since I installed the two bank setup (three years). The plan was to have some physical restriction to turning both switches on at the same time.
>
> If one bank was depleted and someone turned on the other bank without turning off the depleted bank's switch any idea on how fast would the current would flow to the low SOC bank? Seems it could happen pretty quickly and damage some batteries.
>
> Thanks again,
>
> Mark
> Santa Cruz
>
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